1. Field of the Invention
The present invention relates to a recording method and apparatus of a magneto-optical recording medium, in which reproduction of a recording magnetic domain is executed by displacing a domain wall of the recording magnetic domain in a domain wall displacement layer (hereinafter, referred to as “displacement layer”) by light beam irradiation while not varying recorded data in a memory retaining layer (hereinafter, referred to as “memory layer”) and detecting a change in a polarization plane of a reflected light of the light beam.
2. Related Background Art
For writable high-density recording, there is known a magneto-optical recording medium in which information is recorded by writing a magnetic domain in a magnetic thin film by thermal energy of a semiconductor laser and information is read out by the magneto-optical effect. It is recently requested to further increase the recording density of such magneto-optical recording medium to obtain a large-capacity recording medium.
The linear recording density of an optical disk such as a magneto-optical recording medium depends largely on the wavelength of the laser of a reproducing optical system and a numerical aperture of an objective lens. More specifically, once the laser wavelength λ of the reproducing optical system and a numerical aperture NA of an objective lens are fixed, a diameter of a beam waist is determined, so that the detection limit of the spatial frequency at the reproduction of the recording magnetic domain becomes about 2NA/λ. Consequently, in order to achieve a higher density in the conventional optical disk, it is necessary to reduce the wavelength of the laser of the reproducing optical system and to increase the numerical aperture NA of the objective lens. However, improvements in the wavelength of the laser and in the numerical aperture of the objective lens also have limitations. For this reason, there are being developed technologies for improving the recording density by improvements in the configuration of the recording medium and in the reading method.
For example, Japanese Patent Application Laid-open No. 06-290496 proposes a method and an apparatus for signal reproduction, after recording a signal in a memory layer in a multi-layered film including a displacement layer, a switching layer and a memory layer which are magnetically coupled, by displacing a domain wall of a recording magnetic domain in the displacement layer without varying data recorded in the memory layer, while utilizing a temperature slope generated on a recording medium by irradiation of a light beam for heating, magnetizing the displacement layer in such a manner that a substantially entire area of a reproducing light beam spot is uniformly magnetized, and detecting a change in a polarization plane of a reflected light of the reproducing light beam, thereby reproducing a recording magnetic domain equal to or smaller than a diffraction limit of the optical system.
This method allows to reproduce a recording magnetic domain of a periodicity equal to or smaller than the diffraction limit of the optical system, thereby providing a magneto-optical recording medium capable of significantly improving the recording density and the transfer rate, and a reproducing method thereof.
However, in the above-described magnetic wall displacing reproduction method in which the signal is reproduced by displacing the domain wall utilizing a slope in the temperature distribution formed on the recording medium, the state of the formed temperature distribution in an area of a non-uniform structure such as a preformatted area is different from that in a data area, and such difference may result in a defective data reproduction. More specifically, since the preformatted area does not have a grooved portion, this is, the preformatted area has no effect of a heat insulating effect by a wall of the groove, the heat is diffused and induces an insufficient temperature elevation, whereby a temperature distribution with a proper temperature slope cannot be formed. As a result, there is a problem that the displacement of the domain wall cannot be achieved properly and a defective data reproduction may result.